Feature/Facility 9 - Atlantic saltmarsh
Description
This feature covers pioneer marsh, atlantic salt meadows and Mediterranean and thermo-Atlantic
halophilous scrub communities.
Salicornia and other annuals colonising mud and sand
SM8 Annual Salicornia (samphire) marsh.
SM9 Sueda maritima (annual seablite) saltmarsh
Atlantic Salt Meadows
SM10 Transitional low marsh vegetation
SM11 Aster tripolium var. discoides (unrayed sea aster) saltmarsh
SM13 Puccinellia maritima (sea meadow grass) saltmarsh
SM14 Halimione portulacoides (sea purslane) saltmarsh
SM16 Festuca rubra (red fescue grass) saltmarsh
SM17 Artemesia maritima (sea wormwood) community
Mediterranean and thermo-Atlantic halophilous scrub
SM21 Suaeda vera - Limonium binervosum (shrubby seablite - rock sea lavender)
saltmarsh
SM25 Suaeda vera saltmarsh (shrubby seablite) saltmarsh
All stages of saltmarsh development are exhibited on the reserve from the colonisation
of bare mud
through to upper marsh communities. At the present time, there are few areas of extensive pioneer
marsh.
Typically, saltmarsh development occurs between the semi- parallel dune ridges on
this stretch of
coast. Irrigation occurs via tidal creeks which drain to the south. The largest such creek is
Greenshank Creek, which drains the extensive saltmarsh system at the north end of the SSSI.
Ultimate transition of saltmarsh through a brackish phase and into freshwater marsh
has been
largely curtailed by the building of Bulldog Bank. However there are regionally rare examples of
such transition, on a small scale, where the East Dune slacks meet the upper saltmarsh.
The Old Saltmarsh (compts 2 and 3) commenced development some 300 years ago, situated
between the East and West Dunes. This exhibits the oldest upper marsh communities and as
such is a rare resource on the Wash where saltmarsh reclamation has been extensive.
Examples of lower saltmarsh communities can still be found here, due to the presence
of low tidal
pans and other depressions which may relate to past military activity. Typical pioneer flora
includes Samphire Salicornia europaea, annual sea blite Sueda maritima and rice
grass Spartina
anglica. The middle marsh plateaux on this marsh, are floristically rich (in saltmarsh terms),
with
most of the typical East Coast halophytes including carpets of common sea lavender Limonium
vulgare. Succession inevitably leads to the encroachment of sea couch Elytrigia atherica
into the
lavender marsh, however this is compensated for through the accretion of the New Saltmarsh where
an extensive lavender marsh is developing.
Dune ridges bisect saltmarsh and provide interesting transition. Along the east dune/saltmarsh
interface, lesser centaury Centaurium pulchellum is found alongside rock sea lavender Limonium
binervosum and Danish scurvy grass Cochlearia danica. The latter two species are better
represented along the edges of the strip saltings to the north where sea heath Frankaenia laevis
is
also abundant. The L binervosum community extends well up to Seacroft where long-bracted
sedge Carex extensa is locally frequent in the sward. Shrubby sea blite Suaeda vera
occurs
patchily in upper saltmarsh around the reserve and as with sea heath, Gibraltar Point is the
northern most limit of their UK range.
Favourable Status
Where the system is actively accreting, and the full range of NVC communities are
represented
within a diverse age structure.
Factors &Limits
1. Continued saltmarsh
development is dependent upon the continued deposition of fine mud
and silts which are derived from offshore and via the river Steeping. The depth of shore
profile/topography is fundamental to the settlement of this fine material and therefore,
colonisation by pioneer saltmarsh vegetation. In periods of silt deposition, saltmarsh
development and succession have been quite rapid, for example the New Saltmarsh (Compt
1/11/a), situated between the East Dunes and the Outer Ridge. Following the consolidation
of the outer ridge, tide- borne silts were deposited on the original 1960’s beach.
Pioneer
vegetation was well established by the mid 1980’s with sea purslane Halimione potulacoides
and sea aster Aster tripolium becoming more prominent the following decade. The transition
through to middle marsh is completed with the spread of sea lavender, which colonised
during this marsh significantly during the latter part of the 1990’s. Rising sea levels
pose a
threat to future saltmarsh development, indeed, it is predicted that erosion will result from
‘coastal squeeze’; the compression of the intertidal zone.
-
- Target: To monitor the development
of saltmarsh.
- 2. In general, visitors tend to
avoid walking through the saltmarshes. Indeed, visitor
management aims at discouraging this. Saltmarsh vegetation suffers through repeated
trampling and the disturbance of the substrate may deter re-establishment. The beach
access from the ‘Sleeper Track’ across the New Saltmarsh (Compt 12) demonstrates this.
In
some areas light trampling combined with rabbit grazing, along path edges has helped to
suppress sea couch, favouring such species as danish scurvy grass and small centaury. In
other areas of saltmarsh edge, maintenance of bare ground is important for invertebrates.
Disturbance by visitors, may have negative impacts on breeding and wintering birds. Low-
intensity samphire picking occurs around the Wash under commoners rights. However,
commercial ‘harvesting’ has been problematic at Seacroft in past years.
- Target: To note the effects of visitor
pressure to inform management decisions.
3. Saltmarshes are capable of locking
up a wide range of pollutant materials with deposited
silt and mud. Research is presently being carried out into volumes and types of heavy
metal. Any form of oil or chemical pollution from offshore vessels could be devastating to the
saltmarsh habitat and dependent flora and fauna.
- Target: To encourage research into
saltmarsh pollution.
4. Livestock grazing is not practised
on the saltmarshes of the NNR. Elsewhere in the Wash
cattle grazing under varied regimes maintains saltmarsh for its floristic interest, grazing
wildfowl during the winter or breeding waders. In places at Gibraltar Point rabbit populations
are high. Rabbit grazing maintains short lawns of sea meadow grass, annual sea blite and
greater sea spurrey, along the margins of the New Saltmarsh. Brent geese seek out these
Puccinellia lawns in the winter months. Along the strip saltings, heavy rabbit grazing favours
such prostrate species as the sea heath.
- Target: To measure the rabbit population
through monthly survey and initiate control
measures as necessary.
5. Mid-upper level saltmarsh supports
a low diversity of nesting birds, but populations of
skylark and meadow pipit can be dense. Nesting cycles have to be brief to avoid clutch
losses to tidal inundation. The fragile existence of such birds is further demonstrated on the
Old Saltmarsh, where many pairs nest within feet of the main visitor access and must tolerate
the regular passage of visitors and dogs. Other birds of conservation concern which nest here
include reed bunting, linnet and occasional grey partridge and lapwing. Redshank favour
dense clumps of sea purslane and sea couch and little terns and ringed plover may move
their chicks into the saltmarsh to conceal them.
- Target: To carry out annual breeding
bird census on two saltmarsh plots.
6. The sparsely vegetated pioneer
saltmarshes are regularly inundated by tides and are fairly
inhospitable environments for terrestrial life. The range of marine invertebrates such as
crustaceans and molluscs provide a food source for shelduck and waders. Brent geese seek
out the Puccinellia lawns. During periods of low visitor numbers, these waders and
wildfowl
will exploit areas of new saltmarsh adjacent to major access points. The seeds from
samphire and annual sea blite provide an abundance of food to sustain winter flocks of
passerines eg shorelark and snow bunting. The reserve is one of the principal UK sites for
wintering shorelark, whilst snow bunting numbers are now irregular and flocks of twite are no
longer a regular feature. Other wintering species which depend upon saltmarsh include large
numbers of rock pipit (presumed of continental origin) and a roost of corn buntings (origins
unknown). Passerine flocks and small mammal populations attract birds of prey.
Target: To survey the saltmarsh wintering
bird populations.
7. In addition to regular short-eared
owls and merlin, hen harriers use the reserve as a
hunting ground. Hen harriers have also been attracted into a winter roost, on or adjacent to
saltmarsh on the Steeping Haven. One to five birds have been regular between Nov and
March, but the proximity of wildfowler activity may limit use of the roost.
- Target: To carry out winter surveys
of the roost and record disturbance factors.
8. Sea wormwood Artemesia
maritima is the sole food plant of the rare scarce pug moth. It
is found patchily in the upper marsh, where tidal inundation occurs only on spring tide
sequences. Whilst it appears to persist well within dense sea couch grass, ultimately it may
be out- competed. The disturbance of an area of couch adjacent to the Field Station has
enabled sea wormwood to become established following the installation of an outflow pipe.
Target: To measure the extent of
sea wormwood to inform management decisions.
9. In addition to the scarce pug,
saltmarsh plants support other coastal lepidoptera of
restricted range. For example, the abundant sea aster hosts the nationally notable star- wort
moth Cucculia asteris. Sea meadow grass sustains populations of the crescent striped
Apamea oblongaand Pediasia aridella.
Target: To census the saltmarsh lepidoptera
assemblage through light-trapping at least
once per month in suitable conditions May-Aug.
10. At the upper reaches of the Old
Saltmarsh, there are interesting transition zones with the
dune systems, particularly where rabbit grazing and trampling suppress sea couch and
ensure a low sward and bare ground. The bare ground and cliffing along the West dune edge
shows an extensive colony of solitary wasps; another remarkable invertebrate family capable
of dealing with occasional tidal submersion. A notable species is Halictus rubicundus.
Target: To survey the condition of
the solitary wasp colonies.
11. Ungrazed upper saltmarsh is a
scarce habitat which is attractive to a range of
invertebrates, particularly arachnids and Orthoptera. The short-winged conehead
Conocephalus dorsalis is one example. This species inhabits the rank couch along
the
saltmarsh margins.
Target: To record the presence of
short-winged conehead in saltmarsh annually.
12. Past colonisation of the upper,
drier reaches of the Old Saltmarsh and strip saltings has
occurred by sea buckthorn. Much of this growth however appears to have been killed through
extreme high tides and an unusual legacy of standing deadwood provides variation in this
niche.
- Target: To survey scrub encroachment
along saltmarsh margins.
13. Saltmarshes are important in
flood defence terms in absorbing tidal energy over large
areas of the intertidal zone. Paradoxically, they are easily damaged by the actions of
vehicles which destroy vegetation and interrupt the substrate. Particular problems exist with
trail bikes, quads and four-wheel drive vehicles at Seacroft.
- Target: Any threat to saltmarsh
integrity through physical loss or disturbance must be
identified.
14. Intertidal zones are subject
to deposition of flotsam and jetsam. Much of this comes in
the form of shipping refuse and may be hazardous to visitors and wildlife and may spoil the
aesthetics of the site. Bird carcasses may also be deposited by the tide, particularly after
incidents discussed under factor 3. National surveys such as Coastwatch are undertaken
periodically to establish the volume and source of marine refuse.
- Target: To survey the strandline
for undesirable refuse/beached birds etc.
15. The tidal pools of the Old Saltmarsh
are infrequently replenished by tidal inundation via
the network of creeks - tides over 7.0m. During the summer, evaporation causes hyper-
saline conditions and panning. Conversely, during the winter, fresher conditions may
predominate through precipitation and head of pressure from freshwater slacks. These
unstable pools, are thus inhospitable to a broad range of aquatic life. However, certain
specialist diptera may exist here and the resultant bare mud is exploited by a range of
coleoptera and hemiptera. Sea club rush at the margins of tidal slacks provides a further
important niche for invertebrates. The network of pans and creeks enables pioneer vegetation
types to persist in maturing saltmarsh.
- Target: To record change in pattern
and extent of pans and creeks via aerial
photography.
Attributes &Limits
- 1. The total extent of saltmarsh
will be measured once during the reporting cycle.
- Target: There should be no decrease
in extent from the 1999 aerial survey.
- 2. The frequency and abundance of
constant species for each community or sub-
community will be measured once during the reporting cycle. The natural dynamism of the
feature will inevitably see change in the distribution of the key communities resulting from
succession.
- Target: No significant change recorded
in the extent and proportion of each NVC
community from the 1999 NVC survey.
- 3. Coastal squeeze may cause saltmarsh
erosion and a landward migration of age classes.
- Target: There should be no artificial
constraint to curb a ‘natural retreat’ situation.
